Magnetic recording media are widely used as audio tapes, video tapes, computer tapes or as floppy disks. A magnetic recording medium basically comprises a magnetic layer containing ferromagnetic particles dispersed in a binder and being laminated on a non-magnetic support.
Fundamentally, a magnetic recording medium is required to have high levels of various characteristics such as electromagnetic characteristics, running durability and running efficiency. Particularly, with the recent extensive development of 8 mm video tape recorders, it is necessary for a video tape to have particularly excellent electromagnetic characteristics such as a high video output and a high ability for reproducing original images.
There are various approaches to improvement of electromagnetic characteristics of a magnetic recording medium. One direct and effective approach is to improve the characteristics of the ferromagnetic particles, which serve as magnetic recording substances. Accordingly, ferromagnetic particles have been granulated finer and finer in order to achieve high density recording. Iron oxide modified with a different metal such as cobalt has been used instead of iron oxide as material for ferromagnetic particles. Recently, ferromagnetic metals such as iron, nickel or cobalt and alloys containing the above metals have been used.
It is possible to obtain a magnetic recording medium having excellent electromagnetic characteristics by using the thus-improved ferromagnetic particles. However, in practice, it is difficult to prepare such a magnetic recording medium. The reason for this difficulty is that sometimes the thus-improved ferromagnetic particles are improved in decrease of dispersibility of these ferromagnetic particles in the magnetic layer. Thus, the excellent characteristics of these ferromagnetic particles cannot be fully realized because as the ferromagnetic particles are made finer and finer, the dispersibility of the ferromagnetic particles in the binder tends to become poorer. The following difficult-to-disperse ferromagnetic particles are listed in descending order of dispersibility: .gamma.-iron oxide, cobalt-coated .gamma.-iron oxide, and ferromagnetic metal fine particles.
In another approach to the preparation of a magnetic coating composition, the steps of mixing, kneading and dispersing are conducted for a long period of time to improve the dispersibility of the above-described difficult-to-disperse ferromagnetic particles. However, this approach has two disadvantages. First, the characteristics of the fine ferromagnetic particles are sometimes deteriorated by the shearing force incident to mixing, kneading and dispersing the particles. Second, this manufacturing process is highly time consuming and, therefore, undesirable in light of production efficiency.
In this connection, methods for effectively dispersing the above-described difficult-to-disperse ferromagnetic particles without drastically changing the conventional process for preparing a magnetic recording medium have been investigated. Known examples of such methods include (i) a method for using ferromagnetic particles surface-treated with surface treating agents such as a silane coupling agent, (ii) a method for using dispersing agents such as a fatty acid to improve the dispersibility of ferromagnetic particles, and (iii) a method for using as a binder a mixture of the system comprising a vinyl copolymer, a urethane prepolymer, and low molecular weight epoxy resins (as disclosed in JP-B-56-23210) (the term "JP-B" as used herein refers to an "examined Japanese patent publication").
However, it has been found by the present inventors that the dispersibility of ferromagnetic particles cannot always sufficiently be improved even by the known methods.
For example, when ferromagnetic particles are surface-treated with the above-described silane coupling agents, the surface of the particles is made hydrophobic by the silane coupling agent, and thus the stability of dispersed ferromagnetic particles in the magnetic coating composition is improved. However, the compatibility of the coated ferromagnetic particles with the resins is disadvantageously decreased. This relative incompatibility sometimes prevents the dispersibility of ferromagnetic particles in the magnetic layer from being sufficiently improved at a final stage. Further, the silane coupling agent itself is very expensive, and, therefore, is difficult to use as a treating agent for treating ferromagnetic particles used in a conventional magnetic recording medium in light of the manufacturing costs.
A fatty acid which is generally employed as a lubricating agent in the magnetic layer of a magnetic recording medium has the ability to disperse ferromagnetic particles. The dispersibility of ferromagnetic particles can be improved by adjusting the added amount thereof. Generally, the amount of fatty acid sufficient for use as a dispersing agent should be more than the amount used when the fatty acid is employed as a lubricating agent in the magnetic layer.
On the other hand, it is known that when a fatty acid is used in an excess amount, it works as a plasticizing agent for the binder. Therefore, when the fatty acid is used as a dispersing agent for ferromagnetic particles, the fatty acid is inevitably used in excess and the binder is therefore inevitably plasticized. When the binder is plasticized, running properties deteriorate. Thus, even though the surface smoothness of a magnetic recording medium is improved by improving the dispersibility of the ferromagnetic particles contained in the medium's magnetic layer, the running properties are deteriorated. In other words, although dispersibility can be improved by using a large amount of fatty acid, durability and running properties can suffer.
As described above, a low molecular weight epoxy resin can be mixed with a vinyl copolymer and a urethane prepolymer to form a predetermined binder which can be mixed and kneaded with ferromagnetic particles and coated. In this case, not only is the binder especially predetermined, but also the improvement of the dispersibility of the ferromagnetic particles is insufficient, and as described above, the great length of time required for mixing and kneading deteriorates the characteristics of the ferromagnetic particles.